The present invention relates to a method for preparing a dispersion of ZnS particles doped with copper ions showing enhanced luminescence.
Luminescence of sulfides, selenides and tellurides of Zn, Sr, Cd, Hg, and others, is due to the presence of luminescent centres in the host crystal. For instance, Cu+ centres in ZnS give rise to luminescence at 520 nm. Mn2+ is also a luminescent centre for ZnS. The emission maximum in this case is situated at about 590 nm.
The method that is usually applied for the preparation of (electro) luminescent particles is the sintering method. In this procedure bulk ZnS and e.g. MnS are mixed and sintered at high temperature ( greater than 700xc2x0 C.) under H2S atmosphere. As a result the Mn2+ ions diffuse in the ZnS lattice. In a next step the resulting ZnS:Mn pieces are milled to fine particles. The disadvantages of this method are the high temperature needed, the H2S atmosphere, and the large final average particle size (5-20 xcexcm).
Recently, scientific literature(1-9) mentions luminescent ZnS:Mn nanoparticles prepared according to the homogeneous precipitation method. This method allows the building in of luminescent Mn2+ centres in a simple way. In this method a homogeneous solution containing Zn2+ and Mn2xe2x88x92 ions is mixed with a solution of sulfide ions. In this way luminescent nanoparticles of ZnS:Mn originate.
The solubility of ZnS (sfalerite) in cold water is 6.9xc3x9710xe2x88x924 g/l while the solubility of MnS is 4.7xc3x9710xe2x88x923 g/l, which makes no big difference. Moreover, MnS is well homogeneously mixable with ZnS up to 40 mol %. As a result doped ZnS:Mn particles can be simply prepared according to the homogeneous precipitation method.
The situation is completely different when one intends to build in Cu+ ions. Doping of ZnS with Cu+ gives rise to emission at 485 nm. The solubility of Cu2S in cold water is 10xe2x88x9213 g/l which is several orders of magnitude lower than the one of ZnS. Also the solubility of Cu+ in a ZnS lattice is limited. The maximal concentration amounts to only 0.05% vis-à-vis the ZnS. Therefore, when using the homogeneous precipitation method, there is a big chance that a separate phase of Cu2S nuclei or particles is formed apart from the ZnS. A way of preparing yet luminescent ZnS:Cu nanoparticles uses the heterogeneous precipitation method. In this method one uses micellar particles in petroleum ether filled with aqueous solutions of Zn2+ and Cu+ on the one hand and S2+ on the other hand. Apparently the kinetics and/or thermodynamics of the precipitation are changed in such a way that luminescent ZnS:Cu particles are still formed.
Another way is to lean only on the kinetics. Cu+ can form a complex, so that the the Cu+ ions are only slowly liberated in the precipitation reaction. As a result isolated building in of Cu+ in ZnS becomes possible. An example is described by Sun(10): in this case thiourea or thiosulphate are used as complexants.
The present invention extends the teachings on the preparation of luminescent ZnS:Cu particles.
References
(1) Eshuis A.; van Elderen G. R. A.; Koning C. A. J.; Colloids and Surfaces A: Physicochemical and Engineering Aspects (1999), 151, 505-512.
(2) Gallagher, D.; Heady, W. E.; Racz, J. M.; Bhargava, R. N.; J. Mater. Res. (1995), 10(4), 870-6.
(3) Murase, N.; Jagannathan, R.; Kanematsu, Y.; Watanabe, M.; Kurita, A.; Hirata, K.; Yazawa, T.; Kushida, T.; J. Phys. Chem. B (1999), 103(5), 754-760.
(4) Vacassy, Robert; Scholz, Stefan M.; Dutta, Joydeep; Plummer, Christopher John George; Houriet, Raymond; Hofmann, Heinrich; J. Am. Ceram. Soc. (1998), 81(10), 2699-2705.
(5) Yu, I.; Isobe T.; Senna M.; J. Phys. Chem. Solids (1996), 57(4), 373-379.
(6) Que, Wenxiu; Zhou, Y.; Lam, Y. L.; Chan, Y. C.; Kam, C. H.; Liu, B.; Gan, L. M.; Chew, C. H.; Xu, G. Q.; Chua, S. J.; Xu, S. J.; Mendis, F. V. C.; Appl. Phys. Lett. (1998), 73(19), 2727-2729.
(7) Xu, S. J.; Chua, S. J.; Liu, B.; Gan, L. M.; Chew, C. H.; Xu, C. Q. Appl. Phys. Lett. (1998), 73(4), 478-480.
(8) Gan, L. M.,; Liu, B.; Chew, C. H.; Xu, S. J.; Chua, S. J.; Loy, G. L.; Xu, G. Q.; Langmuir (1997), 13(24), 6427-6431.
(9) Leeb, J.; Gebhardt, V.; Mueller, G.; Haarer, D.; Su, D.; Giersig, M.; McMahon, G.; Spanhel, L. Institut fuer Silicatchemie, Universitaet Wuerzburg, Wuerzburg, Germany. J. Phys. Chem. B (1999), 103(37), 7839-7845.
(10) Sun L.; Liu C.; Liao C.; Yan C.; Solid State Comm. (1999), 111, 483-488.
It is an object of the present invention to provide a method for the preparation of a dispersion of zinc sulfide particles doped with copper which show enhanced luminescence.
It is a further object of the present invention to provide a Thin Film Inorganic Light Emitting Diode device with enhanced luminescence.
The above mentioned objects are realised by providing a method for the preparation of a dispersion of zinc sulfide particles doped with copper (ZnS:Cu), said method comprising the step of performing a precipitation by mixing together a zinc salt, a sulfide, and a citrate or EDTA complex of copper ions, dissolved in several aqueous solutions.
After the precipitation the ZnS:Cu particles are preferably washed, preferably by a diafiltration and/or an ultrafiltration treatment.
The thus obtained ZnS:Cu dispersion can be coated as a layer making part of an (electro)luminescent device such as a Thin Film Inorganic Light Emitting Diode device (ILED).